Automotive Differentials, Release

7/28/2019 Automotive Differentials, Release

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Automotive and Racing Differentials


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Overview Why do you need a differential? Typical Types of Differentials

Brief Overview

Advantages and Disadvantages

Differential Applications Porsche ALMS LMP2 Prototype Differential Configuration

Description of Layout

Adjustability

Influence on Vehicle Performance


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Why do we need a differential? Quantifying a Turn

centerr

outerrcenter

rinnerr


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Assumptions

Pure rolling, no wheel spin Constant yaw velocity, No Yaw Accel.

Known Constants

Vehicle Track Width LTW

Lateral Acceleration ALat Vehicle Speed Vs

Tire Radius rtire

Example:LTW = 72 in.

ALat = 2.0 g

Vs = 60 mph

rtire = 14 in.

Determining Corner Radius:

This set of equations quantifies the action

required by the differential to negotiate a

given corner

%1.5100*

6.17

9.0

9.07.166.17

6.173.7

8.128

7.163.7

5.122

3.712/1422

8.1281232360

602

360

5.1221172360

602

360

123

2

12/72120

2

1172

12/72120

2

120

1202.320.2

4667.160222

slip

innerouterdiff

tire

outerouter

tire

innerinner

tiretire

outerouter

innerinner

TWcenterouter

TWcenterinner

center

Lat

scentersLat

RPM

RPMRPMRPM

C

lRPM

C

lRPM

ftrC

ftrl

ftrl

ftL

rr

ftL

rr

ftr

ftA

Vrr

VA


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Artificial Wheel Spin Percentage

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

0 50 100 150 200 250 300 350 400 450

Corner Radius (ft)

InsideWheelSpin(%)

Artificial Wheel

Spin Percentage


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Options to a Differential Train wheel, the angle of the wheel allows the train tomove laterally and change the rolling radius of the train

wheel

Tire stagger

Utilized in Oval Racing


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Typical Differential Configurations Open Differential

Locked Differential, Spool

Limited Slip Differentials


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Limited Slip DifferentialAdvantages: Manages vehicle traction mechanically

Completely passive system, no user or electrical controller inputrequired

Disadvantages:

Many moving components, more susceptible to failure

Expensive to produce

Applications:

Optional OEM equipment, performance oriented or high endproduction vehicles

Road racing

Types of Limited Slip Differentials:

Salisbury style

Viscous

Torsen


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Salisbury Style Differential Purely mechanical differential which relies on pressure angles to generate a clampingforce on a clutch pack to resist different wheel speeds

Highly adjustable and can be characterized through simple simulation or test

equipment.

Minimal temperature sensitivity

Expensive to manufacture due to all of the components involved, tight tolerances

Small performance and operational window

Components can wear out and the performance of the differential can be adversely


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Viscous Differential Hydro-mechanical differential that relies on the viscous properties ofan oil or grease to generate a resistance to different wheel speeds

Highly influenced by temperature

Semi-adaptive, force is dependent wheel speed difference when

responding to slip. Characterized as a speed sensitive differentialrather than a torque sensing differential.


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Torsen Style Shortened from Torque Sensing Differential.

Relies on gear pressure angles to cause a mechanical binding in a geartrain that resists different wheel speeds.

Non adjustable, and difficult to quantify the performance without expensiveequipment.

Has a large performance window, which enables it to be used in a widerange of applications

Still relies on torque input from the wheels to respond and can only apply aratio of the torque to the non-slipping wheel


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Porsche LMP Differential Configuration Differential Layout

Combination of a Salisbury Differential and a Viscous Differential


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Adjustability Optional friction plate material

Sintered Steel Plates

Sintered Bronze

Carbon Matrix

Clutch pack preload Static clutch pack compression with preload springs

Clutch pack configuration

Number of friction faces

Clutch pack pressure angles

Optional coast and drive pressure angles Optional Viscous Differential Stiffness


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Influence of Optional Friction Materials


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Influence of Clutch Pack Preload


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Clutch Pack Configuration Increasing the number of friction facesincreases the area of the friction surface

Increasing the friction area translates a

larger portion of the differential clamping

load into differential force, giving a tighter

differential


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Optional Viscous Coupling Stiffness

To change the Viscous coupling stiffness the entire unit must be changed or modified.The stiffness of the coupling is adjusted by changing the amount of sheering plates or the

viscosity properties of the oil or grease


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Influence of the Viscous Coupling on Total Differential Force


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Simple Representation:

.deg45

300

300300

45tantan

lbsF

lbslbsF

F

Torque

Torque

Clamp


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Influence of the Differential on Vehicle Dynamics Yaw stability is commonly referred to as vehicle balance, the differential can

have a large influence on the yaw stability of the vehicle.

Understeer Balance Condition, Tight

Nuetral Balance Condition

Oversteer Balance Condition, Loose


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Braking Zone During this phase of the corner the differential will have the leastamount of influence. The rear brakes are determining the speed

and torque of the rear wheels and at the same time the torque

coming from the engine is small as well, engine braking only. In a

symmetric left to right brake bias setup the forces from the brakes

will be equal. The influence of the differential increases progressively as the brake

pressure decreases

The differential influence will increase as the inertia of the engine

components increases


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Braking Zone Forces

RFyF

RFxF

xD

FRONTyD

REARyD

CGM

LFyF

LFxF

RRxF

LRyF

LRxF

RRyF

+Y

+X

DIFFyF


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Resolved Moment

xxX

yyY

DFM

DFM

LFLF

FRONTLFLF

Moment Contribution LF Moment Contribution RF

xxX

yyY

DFM

DFM

RFRF

FRONTRFRF

Moment Contribution LR

xDiffxXyyY

DFFM

DFM

LRLR

FRONTLRLR

Moment Contribution RR

xxX

yyY

DFM

DFM

RRRR

FRONTRRRR

LRLRRRRRRFLFRFRF xyxyxyXyCGMMMMMMMMM

Resolved Moment for a Left Hand Turn:


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Mid-Corner Rolling Phase The middle of the corner is where the yaw velocity is at its highest, and there is

little or no influence from the throttle or brakes, zero longitudinal acceleration

RFxF

xD

FRONTyD

REARyD

CGM

LFxF

RRxF

LRxF

+Y

+X

DIFFyF


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Corner Exit On Throttle Phase

RFxF

xD

FRONTyD

REARyD

CGM

LFxF

RRxF

LRxF

RRyF

+Y

+X

RRDIFF yyFF


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Questions


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Quiz1) What is the not a reason for having a differential in your car?

a) Increase tire life

b) Decrease the turning radius to help the car turn in a

parking lot

c) Accommodate the two different arcs carved by the car

through a turn

d) All of the above

e) None of the above

2) Which differential will respond to wheel speed differences?

a) Salisbury Differential

b) Viscous Differential

c) Open Differential

d) Torsen Differential


3) Which differential has the largest performance window?

a) Salisbury Differential

b) Torsen Differential

c) Viscous Differential

d) Open Differential


4) During which phase of the corner does the differential have the

lease amount of influence?

a) Corner Entry

b) Mid Corner

c) Corner Exit

d) The Entire Corner

e) None of the Corner

5) If the driver is complaining about poor acceleration out of the

corner, what adjustment to the differential would you make tohelp fix the drivers problem?

a) Increase the differential force to decrease inside wheel

spin

b) Decrease the differential force to increase the inside

wheel spin

c) Do nothing and tell the driver to quit complaining and

push the gas pedal harder

d) The differential can do nothing to help solve the

handling issue

Automotive Differentials, Release

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